Yuma County is a county located in the southwestern corner of the U.S. state of Arizona. As of the 2010 census, its population was 195,751. The county seat is Yuma.Yuma County comprises the Yuma, AZ Metropolitan Statistical Area.The county's southwestern Colorado River border forms a four-state division between southeast California and southwest Arizona, and northeast Baja California state, Mexico with northwest Sonora state.